At the main menu, go to Options and toggle the developer console on. Now you'll be able to open your the console by pressing ~ (or the key above Tab).

Type sv_cheats 1 in the console. This will enable the cheats. When you get the message "sv_cvar_cheats_changed_to 1," type in the following codes for the desired effect. Note: these cheats are mainly from Left for Dead, and may not work in the sequel.